Sulfolobus is a genus of Gram-negative bacteria — aerobic or facultatively anaerobic, coccoid or irregularly shaped — in the family Sulfolobaceae, found in sulfur-rich hot springs. These extremophiles thrive near boiling point in acidic waters, and Sulfolobus solfataricus is used in the production of several thermostable enzymes, such as thermostable beta-glucosidases, prized for starch processing and other high-temperature biocatalysis. Their enzymes’ robustness illustrates how life at the extremes supplies biotechnology with catalysts that survive conditions destroying ordinary proteins. Their study also illuminates the limits of life and the origins of key metabolic pathways.